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Lecture 5

sed and awk

Last week

• Regular Expressions– grep

– egrep

Today

• Stream manipulation:– sed

– awk

Sed: Stream-oriented, Non-Interactive, Text Editor

• Look for patterns one line at a time, like grep

• Change lines of the file

• Non-interactive text editor– Editing commands come in as script

– There is an interactive editor ed which accepts the samecommands

• A Unix filter– Superset of previously mentioned tools

Conceptual overview

• All editing commands in a sed script are applied inorder to each input line.

• If a command changes the input, subsequentcommand address will be applied to the current(modified) line in the pattern space, not the originalinput line.

• The original input file is unchanged (sed is a filter),and the results are sent to standard output (but canbe redirected to a file).

Sed Architecture

scriptfile

Input

Output

Input line(Pattern Space)

Hold Space

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Scripts• A script is nothing more than a file of commands• Each command consists of up to two addresses and

an action, where the address can be a regularexpression or line number.

address action command

address action

address action

address action

address action

script

Scripts (continued)

• As each line of the input file is read, sed reads thefirst command of the script and checks the addressagainst the current input line:– If there is a match, the command is executed– If there is no match, the command is ignored– sed then repeats this action for every command in the

script file

• When it has reached the end of the script, sedoutputs the current line (pattern space) unlessthe -n option has been set

Sed Flow of Control• sed then reads the next line in the input file and

restarts from the beginning of the script file• All commands in the script file are compared to,

and potentially act on, all lines in the input file

. . .cmd 1 cmd ncmd 2

script

input

outputoutput

only without -n

print cmd

sed Commands• sed commands have the general form

– [address[, address]][!]command [arguments]

• sed copies each input line into a pattern space– If the address of the command matches the line in the

pattern space, the command is applied to that line– If the command has no address, it is applied to each line

as it enters pattern space– If a command changes the line in pattern space,

subsequent commands operate on the modified line

• When all commands have been read, the line inpattern space is written to standard output and anew line is read into pattern space

Addressing

• An address can be either a line number or apattern, enclosed in slashes ( /pattern/ )

• A pattern is described using regularexpressions (BREs, as in grep)

• If no pattern is specified, the command willbe applied to all lines of the input file

• To refer to the last line: $

Addressing (continued)

• Most commands will accept two addresses– If only one address is given, the command operates

only on that line– If two comma separated addresses are given, then the

command operates on a range of lines between the firstand second address, inclusively

• The ! operator can be used to negate an address,ie; address!command causes command to beapplied to all lines that do not match address

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Commands

• command is a single letter• Example: Deletion: d•[address1][,address2]d

– Delete the addressed line(s) from the patternspace; line(s) not passed to standard output.

– A new line of input is read and editing resumeswith the first command of the script.

Address and Command Examples

• d deletes the all lines• 6d deletes line 6• /^$/d deletes all blank lines• 1,10d deletes lines 1 through 10• 1,/^$/d deletes from line 1 through the first blank line• /^$/,$d deletes from the first blank line through

the last line of the file• /^$/,10d deletes from the first blank line through line 10• /^ya*y/,/[0-9]$/d deletes from the first line that begins

with yay, yaay, yaaay, etc. throughthe first line that ends with a digit

Multiple Commands

• Braces {} can be used to apply multiple commands to anaddress

[/pattern/[,/pattern/]]{command1command2command3}

• Strange syntax:– The opening brace must be the last character on a line– The closing brace must be on a line by itself– Make sure there are no spaces following the braces

Sed Commands

• Although sed contains many editing commands,we are only going to cover the following subset:

• d - delete•p - print• y - transform• q - quit

• s - substitute• a - append• i - insert• c - change

sed Syntax• Syntax: sed [-n] [-e] [‘command’] [file…]

sed [-n] [-f scriptfile] [file…]– -n - only print lines specified with the print command

(or the ‘p’ flag of the substitute (‘s’) command)

– -f scriptfile - next argument is a filename containingediting commands

– -e command - the next argument is an editing commandrather than a filename, useful if multiple commands arespecified

– If the first line of a scriptfile is “#n”, sed acts as though-n had been specified

Print

• The Print command (p) can be used to force thepattern space to be output, useful if the -n optionhas been specified

• Syntax: [address1[,address2]]p• Note: if the -n or #n option has not been specified,p will cause the line to be output twice!

• Examples:1,5p will display lines 1 through 5/^$/,$p will display the lines from the first blank line through the last line of the file

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Substitute

• Syntax:[address(es)]s/pattern/replacement/[flags]– pattern - search pattern

– replacement - replacement string for pattern

– flags - optionally any of the following• n a number from 1 to 512 indicating which

occurrence of pattern should be replaced

• g global, replace all occurrences of pattern in pattern space

• p print contents of pattern space

Substitute Examples

• s/Puff Daddy/P. Diddy/– Substitute P. Diddy for the first occurrence of Puff Daddy in

pattern space

• s/Tom/Dick/2– Substitutes Dick for the second occurrence of Tom in the

pattern space

• s/wood/plastic/p– Substitutes plastic for the first occurrence of wood and

outputs (prints) pattern space

Replacement Patterns

• Substitute can use several special charactersin the replacement string– & - replaced by the entire string matched in the

regular expression for pattern– \n - replaced by the nth substring (or

subexpression) previously specified using “\(“and “\)”

– \ - used to escape the ampersand (&) and thebackslash (\)

Replacement Pattern Examples"the UNIX operating system …"s/.NI./wonderful &/"the wonderful UNIX operating system …"

cat test1first:secondone:twosed 's/\(.*\):\(.*\)/\2:\1/' test1second:firsttwo:one

sed 's/\([[:alpha:]]\)\([^ \n]*\)/\2\1ay/g'– Pig Latin ("unix is fun" -> "nixuay siay unfay")

Append, Insert, and Change

• Syntax for these commands is a little strangebecause they must be specified on multiple lines

• append [address]a\ text• insert [address]i\ text• change [address(es)]c\ text• append/insert for single lines only, not range

Append and Insert• Append places text after the current line in pattern space• Insert places text before the current line in pattern space

– Each of these commands requires a \ following it.text must begin on the next line.

– If text begins with whitespace, sed will discard itunless you start the line with a \

• Example:/<Insert Text Here>/i\

Line 1 of inserted text\ \ Line 2 of inserted text would leave the following in the pattern space

Line 1 of inserted text Line 2 of inserted text

<Insert Text Here>

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Change

• Unlike Insert and Append, Change can be appliedto either a single line address or a range ofaddresses

• When applied to a range, the entire range isreplaced by text specified with change, not eachline– Exception: If the Change command is executed with

other commands enclosed in { } that act on a range oflines, each line will be replaced with text

• No subsequent editing allowed

Change Examples

• Remove mail headers, ie;the address specifies a rangeof lines beginning with aline that begins with Fromuntil the first blank line.– The first example replaces all

lines with a single occurrenceof <Mail Header Removed>.

– The second example replaceseach line with <Mail HeaderRemoved>

/^From /,/^$/c\ <Mail Headers Removed>

/^From /,/^$/{s/^From //pc\<Mail Header Removed>}

Using !

• If an address is followed by an exclamation point(!), the associated command is applied to all linesthat don’t match the address or address range

• Examples: 1,5!d would delete all lines except 1 through 5 /black/!s/cow/horse/ would substitute

“horse” for “cow” on all lines except those thatcontained “black”

“The brown cow” -> “The brown horse”“The black cow” -> “The black cow”

Transform• The Transform command (y) operates like tr, it

does a one-to-one or character-to-characterreplacement

• Transform accepts zero, one or two addresses• [address[,address]]y/abc/xyz/

– every a within the specified address(es) is transformedto an x. The same is true for b to y and c to z

– y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/ changes all lower case characters on theaddressed line to upper case

– If you only want to transform specific characters (or aword) in the line, it is much more difficult and requiresuse of the hold space

Pattern and Hold spaces

• Pattern space: Workspace or temporarybuffer where a single line of input is heldwhile the editing commands are applied

• Hold space: Secondary temporary bufferfor temporary storage only

Pattern

Hold

in

out

h, H, g, G, x

Quit• Quit causes sed to stop reading new input lines

and stop sending them to standard output• It takes at most a single line address

– Once a line matching the address is reached, the scriptwill be terminated

– This can be used to save time when you only want toprocess some portion of the beginning of a file

• Example: to print the first 100 lines of a file (likehead) use:– sed '100q' filename– sed will, by default, send the first 100 lines of filename

to standard output and then quit processing

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Sed Advantages

• Regular expressions

• Fast

• Concise

Sed Drawbacks

• Hard to remember text from one line toanother

• Not possible to go backward in the file

• No way to do forward references like/..../+1

• No facilities to manipulate numbers

• Cumbersome syntax

Awk

Programmable Filters

Aho Weinberger Kernighan

Why is it called AWK?

Awk Introduction• awk's purpose: A general purpose programmable

filter that handles text (strings) as easily as numbers– This makes awk one of the most powerful of the Unix

utilities

• awk processes fields while sed only processes lines• nawk (new awk) is the new standard for awk

– Designed to facilitate large awk programs– gawk is a free nawk clone from GNU

• awk gets it’s input from– files– redirection and pipes– directly from standard input

AWK Highlights• A programming language for handling common

data manipulation tasks with only a few lines ofcode

• awk is a pattern-action language, like sed• The language looks a little like C but

automatically handles input, field splitting,initialization, and memory management– Built-in string and number data types– No variable type declarations

• awk is a great prototyping language– Start with a few lines and keep adding until it does what

you want

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Awk Features over Sed

• Convenient numeric processing

• Variables and control flow in the actions

• Convenient way of accessing fields withinlines

• Flexible printing• Built-in arithmetic and string functions

• C-like syntax

BEGIN {action}

pattern {action}

pattern {action}

.

.

.

pattern { action}

END {action}

Structure of an AWK Program

• An awk program consists of:– An optional BEGIN segment

• For processing to execute prior toreading input

– pattern - action pairs• Processing for input data

• For each pattern matched, thecorresponding action is taken

– An optional END segment• Processing after end of input data

Running an AWK Program

• There are several ways to run an Awk program– awk 'program' input_file(s)

• program and input files are provided as command-linearguments

– awk 'program'• program is a command-line argument; input is taken from

standard input (yes, awk is a filter!)

– awk -f program_file input_files• program is read from a file

Patterns and Actions

• Search a set of files for patterns.

• Perform specified actions upon lines orfields that contain instances of patterns.

• Does not alter input files.

• Process one input line at a time• This is similar to sed

Pattern-Action Structure• Every program statement has to have a pattern or an

action or both

• Default pattern is to match all lines

• Default action is to print current record

• Patterns are simply listed; actions are enclosed in { }

• awk scans a sequence of input lines, or records, oneby one, searching for lines that match the pattern– Meaning of match depends on the pattern

Patterns• Selector that determines whether action is to be

executed• pattern can be:

– the special token BEGIN or END– regular expression (enclosed with //)– relational or string match expression– ! negates the match– arbitrary combination of the above using && ||

• /NYU/ matches if the string “NYU” is in the record• x > 0 matches if the condition is true• /NYU/ && (name == "UNIX Tools")

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BEGIN and END patterns

• BEGIN and END provide a way to gaincontrol before and after processing, forinitialization and wrap-up.– BEGIN: actions are performed before the first

input line is read.

– END: actions are done after the last input linehas been processed.

Actions

• action may include a list of one or more C likestatements, as well as arithmetic and stringexpressions and assignments and multiple outputstreams.

• action is performed on every line that matchespattern.– If pattern is not provided, action is performed on every input line

– If action is not provided, all matching lines are sent to standard output.

• Since patterns and actions are optional, actions mustbe enclosed in braces to distinguish them frompattern.

An Example

ls | awk 'BEGIN { print "List of html files:" }/\.html$/ { print }END { print "There you go!" }'

List of html files:index.htmlas1.htmlas2.htmlThere you go!

Variables

• awk scripts can define and use variablesBEGIN { sum = 0 }{ sum ++ }END { print sum }

• Some variables are predefined

Records

• Default record separator is newline– By default, awk processes its input a line at a

time.

• Could be any other regular expression.• RS: record separator

– Can be changed in BEGIN action

• NR is the variable whose value is thenumber of the current record.

Fields

• Each input line is split into fields.– FS: field separator: default is whitespace (1 or more

spaces or tabs)– awk -Fc option sets FS to the character c

• Can also be changed in BEGIN

– $0 is the entire line

– $1 is the first field, $2 is the second field, ….

• Only fields begin with $, variables are unadorned

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Simple Output From AWK

• Printing Every Line– If an action has no pattern, the action is performed to all

input lines• { print } will print all input lines to standard out• { print $0 } will do the same thing

• Printing Certain Fields– Multiple items can be printed on the same output line

with a single print statement– { print $1, $3 }– Expressions separated by a comma are, by default,

separated by a single space when printed (OFS)

Output (continued)

• NF, the Number of Fields– Any valid expression can be used after a $ to indicate

the contents of a particular field– One built-in expression is NF, or Number of Fields– { print NF, $1, $NF } will print the number of

fields, the first field, and the last field in the currentrecord

– { print $(NF-2) } prints the third to last field

• Computing and Printing– You can also do computations on the field values and

include the results in your output– { print $1, $2 * $3 }

Output (continued)

• Printing Line Numbers– The built-in variable NR can be used to print line

numbers– { print NR, $0 } will print each line prefixed with

its line number

• Putting Text in the Output– You can also add other text to the output besides what

is in the current record– { print "total pay for", $1, "is", $2 * $3 }

– Note that the inserted text needs to be surrounded bydouble quotes

Fancier Output

• Lining Up Fields– Like C, Awk has a printf function for producing

formatted output– printf has the form

• printf( format, val1, val2, val3, … )

{ printf(“total pay for %s is $%.2f\n”, $1, $2 * $3) }

– When using printf, formatting is under your control sono automatic spaces or newlines are provided by awk.You have to insert them yourself.{ printf(“%-8s %6.2f\n”, $1, $2 * $3 ) }

Selection• Awk patterns are good for selecting specific lines

from the input for further processing– Selection by Comparison

• $2 >= 5 { print }

– Selection by Computation• $2 * $3 > 50 { printf(“%6.2f for %s\n”, $2 * $3, $1) }

– Selection by Text Content• $1 == "NYU"• $2 ~ /NYU/

– Combinations of Patterns• $2 >= 4 || $3 >= 20

– Selection by Line Number• NR >= 10 && NR <= 20

Arithmetic and variables

• awk variables take on numeric (floatingpoint) or string values according to context.

• User-defined variables are unadorned (theyneed not be declared).

• By default, user-defined variables areinitialized to the null string which hasnumerical value 0.

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Computing with AWK

• Counting is easy to do with Awk$3 > 15 { emp = emp + 1}END { print emp, “employees worked

more than 15 hrs”}

• Computing Sums and Averages is also simple{ pay = pay + $2 * $3 }END { print NR, “employees” print “total pay is”, pay print “average pay is”, pay/NR }

Handling Text

• One major advantage of Awk is its ability tohandle strings as easily as many languages handlenumbers

• Awk variables can hold strings of characters aswell as numbers, and Awk conveniently translatesback and forth as needed

• This program finds the employee who is paid themost per hour:

# Fields: employee, payrate $2 > maxrate { maxrate = $2; maxemp = $1 }

END { print “highest hourly rate:”, maxrate, “for”, maxemp }

String Manipulation

• String Concatenation– New strings can be created by combining old ones { names = names $1 " " }END { print names }

• Printing the Last Input Line– Although NR retains its value after the last input line

has been read, $0 does not { last = $0 }END { print last }

Built-in Functions

• awk contains a number of built-in functions.length is one of them.

• Counting Lines, Words, and Characters usinglength (a poor man’s wc) { nc = nc + length($0) + 1 nw = nw + NF }END { print NR, "lines,", nw, "words,", nc,

"characters" }

• substr(s, m, n) produces the substring of s thatbegins at position m and is at most n characterslong.

Control Flow Statements

• awk provides several control flow statements formaking decisions and writing loops

• If-Then-Else $2 > 6 { n = n + 1; pay = pay + $2 * $3 }

END { if (n > 0) print n, "employees, total pay is",

pay, "average pay is", pay/n else print "no employees are paid more

than $6/hour" }

Loop Control

• While# interest1 - compute compound interest# input: amount, rate, years# output: compound value at end of each year{ i = 1while (i <= $3) {

printf(“\t%.2f\n”, $1 * (1 + $2) ^ i)i = i + 1

}}

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Do-While Loops

• Do Whiledo {

statement1

}

while (expression)

For statements

• For# interest2 - compute compound interest# input: amount, rate, years# output: compound value at end of each year

{ for (i = 1; i <= $3; i = i + 1)printf("\t%.2f\n", $1 * (1 + $2) ^ i)

}

Arrays

• Array elements are not declared

• Array subscripts can have any value:– Numbers

– Strings! (associative arrays)

• Examples– arr[3]="value"– grade["Korn"]=40.3

Array Example# reverse - print input in reverse order by line

{ line[NR] = $0 } # remember each line

END {for (i=NR; (i > 0); i=i-1) { print line[i]}

}

• for loop to read associative array– for (v in array) { … }

– Assigns to v each subscript of array (unordered)– Element is array[v]

Useful One (or so)-liners

• END { print NR }• NR == 10• { print $NF }• { field = $NF } END { print field }

• NF > 4• $NF > 4• { nf = nf + NF } END { print nf }

More One-liners

• /Jeff/ { nlines = nlines + 1 } END { print nlines }• $1 > max { max = $1; maxline = $0 } END { print max, maxline }• NF > 0• length($0) > 80• { print NF, $0}• { print $2, $1 }• { temp = $1; $1 = $2; $2 = temp; print }• { $2 = ""; print }

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Even More One-liners• { for (i = NF; i > 0; i = i - 1)

printf(“%s “, $i) printf(“\n”) }• { sum = 0 for (i = 1; i <= NF; i = i + 1)

sum = sum + $i print sum }• { for (i = 1; i <= NF; i = i + 1)

sum = sum $i } END { print sum }}

Awk Variables

• $0, $1, $2, $NF

• NR - Number of records processed

• NF - Number of fields in current record

• FILENAME - name of current input file

• FS - Field separator, space or TAB by default

• OFS - Output field separator, space by default

• ARGC/ARGV - Argument Count, ArgumentValue array– Used to get arguments from the command line

Operators• = assignment operator; sets a variable equal to a

value or string• == equality operator; returns TRUE is both sides

are equal• != inverse equality operator• && logical AND• || logical OR• ! logical NOT• <, >, <=, >= relational operators• +, -, /, *, %, ^• String concatenation

Built-In Functions• Arithmetic

– sin, cos, atan, exp, int, log, rand, sqrt

• String– length, substr, split

• Output– print, printf

• Special– system - executes a Unix command

• system(“clear”) to clear the screen• Note double quotes around the Unix command

– exit - stop reading input and go immediately to the ENDpattern-action pair if it exists, otherwise exit the script

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